The present invention relates to prosthetic joints, and particularly to a prosthesis for the knee joint.
Implantable knee prostheses for diseased and/or damaged knees typically include three components, namely a femoral component, a tibial component and a meniscal component. The femoral component may also include a patellar element, or a separate patellar component may be provided. The prosthesis components are generally configured to restore or emulate as much of the natural motion of the knee joint as possible. The selection of the particular prosthesis components is usually dictated by the condition of the patient's knee. For instance, the condition of the distal end of the femur and proximal end of the tibia, as well as the patency of the surrounding ligaments and soft tissue can affect the form of the joint prosthesis.
Generally, a total knee joint replacement includes a tibial component having a platform portion which replaces the entire superior surface of the tibial plateau and substitutes for the tibial condylar surfaces. The femoral component also includes laterally-spaced condylar portions joined by an intercondylar bridge and a patellar surface.
The tibial component typically includes a tibial tray and stem for surgical attachment to the proximal end of the tibia. The component also includes an intermediate articulating surface member that is connected to the tibial tray. The intermediate member defines a bearing surface for articulation of the femoral component thereon. The mating surfaces are smoothly curved in the anterior-posterior (AP) direction to generally match the lateral profile of the natural femoral and tibial condyles, and to ultimately replicate the normal joint movement.
This normal joint movement includes a translational component in the AP direction, as well as a rolling of the femoral condyles on the tibial condyles when the knee is flexed. In addition, the natural tibia is capable of rotation relative to the femur about the axis of the tibia. Thus, an ideal knee prosthesis will be able to achieve all three degrees of freedom of movement. In some cases, the patient's knee lacks adequate posterior support due to a deficient posterior cruciate ligament. In these cases, the modular knee is preferably posteriorly stabilized, meaning that posterior movement of the tibia relative to the femur is restricted. This posterior stabilization can be achieved in a typical implant by a projection or eminence on the tibial insert that engages a box-like intercondylar portion of the femoral component. Intact collateral ligaments keep the projection within the box-like portion as the knee is flexed to inhibit dislocation of the joint at hyper-extension or hyper-flexion.
In order to increase the lifetime of the prosthetic knee joint, the mating bearing surfaces between the tibial and femoral components generally permit a combination of rolling and translational movement as the knee joint is flexed. These two degrees of freedom of movement change the direction of forces between the two components so the force transmitted through the joint is not focused on one location. In response to this optimum design aspect, some prosthetic knees include a translating intermediate bearing component. One problem with modular implants of this type is that the articulating and sliding components can be exposed to the soft tissue surrounding the joint.